Transmission system



c Oct. 28, 1941. w, IL N 2,260,374

TRANSMISSION SYSTEM Filed Aug. 14, 1939 HYBRID SYSTEM INVENTOR. TURNER W. GILMAN ATTORNEYS Patented Oct. 28, 1941 TRANSMISSION SYSTEM Turner W. Gilman, Oak Park, Ill., assignor to Associated Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application August 14-, 1939, Serial No. 290,089

11 Claims.

The present invention relates generally to improvements in signal current transmission systems 0f the type in which signal controlled switching circuits are provided for partially or completely blocking, under certain conditions, certain of the signal current channels included therein and, more particularly, to improvements in telephone substation circuits having incorporated therein coupled signal current channels for the transmission of incoming and outgoing signal currents.

In the usual telephone substation circuit, a hybrid system or antiside tone impedance network is provided for preventing signal currents developed during operation of the transmitter from being transmitted to the receiver and for preventing signal currents incoming over the line extending to the substation from being transmitted to the transmitter. In this type of arrangement one of the factors which determines the efficiency of the side tone suppression is the impedance of the talking circuit established by way of the two connected subscribers lines. This impedance is not the same for any two different connections involving difierent substations and,

accordingly, the hybrid system of each substation is usually balanced to provide maximum side tone.

suppression efiiciency for average line conditions.

In installations wherein amplification of the ina coming and outgoing signal currents is required, the conditions of unbalance introduced into the substation circuit by the impedances of the lines over which an established connection extends may become intolerable. This is particularly true in substation installations provided in an exchange area where the subscribers lines are of widely different lengths. In order to obviate or minimize the singing which may result due to unbalance of the substation circuit occasioned by unfavorable line conditions, signal controlled switching means may be provided in the substation circuit for selectively blocking the channels when not in use. In the usual arrangement of this character the signal channels are completely blocked when not in use. More particularly, during intervals when the transmitting means of the substation is being used to transmit outgoing signal currents, the incoming signal current channel is rendered completely inoperative. Conversely, during those periods when signal currents are incoming to the substation, the transmitting channel is rendered completely inoperative. An arrangement of this character prevents any interruption of a speech train being transmitted in one direction in response to signal currents transmitted in the opposite direction. In other words, conversation break-ins are positively prevented, thus placing a definite limitation on the use of the circuit.

It is an object of the present invention, there'- fore, to provide improved telephone substation apparatus of the character described wherein the blocking of the signal current transmitting and receiving channels is accomplished in a simple and reliable manner and wherein the control circuits are arranged to permit conversation breakms.

It is another object of the invention to provide an improved transmission system particularly suited for use in telephone substation circuits of the character described wherein the two signal current channels are normally active, and improved, exceedingly simple control apparatus is provided for controlling the two channels so that each channel is rendered substantially inactive when the other channel is in use, but is left free to respond to signal currents which would normally traverse the inactive channel.

The invention is illustrated in its embodiment in a substation circuit which is adapted to be connected to an associated telephone line and comprises the usual transmitting and receiving means. The circuit also includes a transmission system which comprises a hybrid or antiside tone network, a first channel for transmitting signal currents from the transmitting means through the hybrid network to the associated line and a second channel for transmitting incoming signal currents from the hybrid network to the receiving means. Each of the two transmission channels has included therein an amplifier which is arranged to be controlled to vary the signal current transmission efficiency or gain of the associated channel. More particularly, a control circuit is provided which is arranged to respond to signal currents traversing one or the other of the two channels to vary the gains through the two amplifiers in inverse senses and in directions determined by the direction of signal current flow through the transmission system. This control circuit comprises electron discharge means including two space current paths and control electrodes for controlling the resistances ofthe paths,

A network including two parallel current paths each comprising a control section and one of the space .current paths connected in series circuit relationship, is provided which is so arranged that the current traversing each of the control sections is variable inversely in accordance with the resistance of the serially related space curcluded in the two channels so that the magnitude I of the current traversing one of the control sections determines the signal current transmission efficiency of one of the transmission channels and the magnitude of the current traversing the other control section determines the signal current transmission efficiency of the other of the two channels. More particularly, the arrangement of the, control network is such that when signal currents traverse one of'the' channels. thetransmissiontefliciencyj of that channel is increased, While the transmission efficiency of the other channel is decreased and Vice versa. t

The novel features believed to be characteristic of the: invention are set forth with particularity in the appended claims, The invention, both as. r to its organization and method of operation, to- I gether with further objects and advantages thereof will best 'be"understood by reference to the-specification taken in connectionwith the singlcfigure of the drawing in which there is illustrated a substation. circuit having incorporated'therein the features of the invention as v V briefly outlined above.

Referring to the drawing the substation circuit there illustrated is adaptedto be connected to a telephone line terminating at the terminals I0 and may, for example, forma part of a conventional automatic or manual telephone, system in which case the distant end thereof will terminate,v

in a line circuit, the character of which is determined by the characterof the exchange. Briefly described, the circuit comprises transmitting means in the. form of a microphone II. and receiving means in the form of aloud speaker 1I2" which, are, adapted to be coupled to an antiside tone networklorhybrid system I3 by means ofvsigna l transmission channels I4 and I5, respectively,

Preferably, 'the loud speaker I2 is of the wellknown moving coil type, having embodied therein a voice orJsignal current coil which is adapted to be energized by incoming signal currents transmitted thereto throughthe hybrid system I3 and,

the signal transmission channel I5.

Briefly described, the outgoing signal current channel comprises a coupling transformer I6,.

an adjustablevoltage dividing signal level control resistor ILavacuum'tube amplifier I8 andacouw pling. transformer I9 connected in tandem in the ordernamed. 'The transmitter II may be of the sound powered type or, if desired, may be off-the 1 type requiring the use of an auxiliary source'fof exciting current. In the particular arrangement shown, the latter type of transmitter is used and,

in order to "supply energizing current to the transmitter-fromthe line terminatingvat the terminals' IILther'ef'isQproyided a direct current circuit which functions: to. block .the transmission; of

signal; currents directly from the transmitter I to]. theiouput side of the channel, I4 while permitting.

direct current toflow from the lineto the trans mitte'r, Thisl circuit includes a high impedance low resistanc ir'on'core choke coil 20 andjtwo low impedance shunt condensers 2| and 22. By virtue of this arrangement, direct current may be forwarded from the line terminals II] through the hybrid system, the secondary winding of the transformer I9, the choke coil 20 and the primary winding of the transformer It to the transmitter I I, but substantially no signal current energy may be transmitted directly from the transmitter II to the output side of the signal current channel I4". The incoming signal current channel I5 comprises an adjustable voltage dividing signal level control resistor 23 and an amplifier 24 connected in tandem in the order named. The two amplifier tubes I8 and 24 respectively included in the outgoing and incoming signal current channels are preferably of the well-known pentodetype employing directly heated filamentary cathodesto insure fast operation of the circuit when the cathode heater circuit is energized. The cathodes of the two tubes are arranged to be energized in parallel over a circuit including the secondary windingv of a voltagev dropping trans former 25. having its primary winding connected to a suitable source of alternating currentthrough the bracketed terminals 26. Anode and screen. potentials may be supplied to the tubes I8 and 24 from a. powerv pack of conventional arrangement, although in the illustrated embodiment 'of the inventionthe anode andscreen current supply source is illustrated as a battery 21 which.

is heavily by-passed for signal current by means of a condenser. 28.

In order to control the signal current transmis-- sion efficiencies of the two channels I4 and I5- by controlling the gain of the two amplifier tubes I8 and 24, there is ,provided a control circuit which. includes an electron discharge device 21, and a resistance network 28'. More particularly the electron discharge tube 21 comprises a cathode 29 of the filamentary type, a pair of anodes 38' and 3I and a pair of control electrodes 32 and 33., This tube may, if desired,beof the charac ter commercially knownas the Type 19 tube. although, if desired, two separate triodes having their cathodes connected in parallel may be used in lieu of the singletube shown. The input electrodes 29 and 33. of this tube are coupled to. the signal transmission channel I5 at a point fol lowing the amplifier 24 over a path which includes avoltage dividing resistor 34' and. a coupling, transformer 35. The input electrodes 29 and 32,

on the other hand, are coupled to the outgoing signal current channel at a point following the amplifier I8 overapath which includes a voltage dividing resistor 36 ,,and a coupling transformer 31. For the purpose of biasing. thecontrolelec-v trodes 32: and 33 negatively with respect to the, associated cathode 29, there is provided a cathode biasing circuit which comprises an adjustable resistor 378' shunted by asignal current by-passcondenser 39-. Current for energizing. the cathode. 29 is delivered thereto over a circuit including the indicated, cathode vbiasing resistor and the, bracketed terminalslil, The resistance network 28 whichisincluded in the output circuit ofthe,

electron discharge tube 2'Iv is essentially a Y -network comprising a base leg 4| in the form ofa voltage dividing-resistor having an adjustable tap- 42, and a pair of resistors 43 and 44 forming the, arms of the Y. The end terminals of the resistors 4|, 43 and 44 are joined by a pair of additionalresistors 45 and 46each having relatively high T resistance values as compared with the resistors, making up the above-described Y'arrangement; ,Preferably, the networkis symmetrical; that is,

the resistance values of the resistors 43 and 44 are equal, being of the order of 5000 ohms each, and the resistance values of the resistors 45 and 46 are equal, being of the order of 10,000 ohms each. The base leg resistor 4| is preferably of the order of 2000 ohms resistance. Anode current is supplied to the anodes 30 and 3| of the tube '2! through the resistance network 28 from a second source of direct current 41, the network being heavily by-passed for signal currents by means of two low impedance condensers 48 and 49.

I With the resistance network 28 arranged in the manner just described, two parallel output current paths are provided through the electron discharge tube 21. One of these paths extends from the positive terminal of the current source 4'! through the resistor 4 the resistor 44, the space current path between the anode 3| and the cathode 29, and the cathode resistor 38 to the negative terminal of the current source 41. The other path extends from the positive terminal 01' the source 4'! through the resistor 4|', the resistor 43, the space current path between the anode 30 and the cathode 29 and through the cathode resistor 38 to the negative terminal of the source 41. The adjustable tap 42 of the resistor 4| is connected through the midpoint of the secondary winding of the transformer 25 to the cathodes of the amplifier tubes I8 and 24. The junction point between the resistors 44 and 46 is connected through the upper portion of the voltage dividing resistor I! to the control electrode of the amplifier tube 8. Similarly, the junction point between the resistors 43 and 45 is connected through the right portion of the voltage dividing resistor 23 to the control electrode of the amplifier tube 24. Thus, the direct voltage drop across the control section of the resistor network which comprises the portion 42b of the resistor 4| and the arm 44 is impressed as a negative bias voltage between the cathode and control electrode of the tube l8, thereby to determine the gain or signal current transmission efficiency of this tube. Similarly, the direct voltage drop across the conrol section of the resistor network which comprises the portion 42b of the resistor 4| and the arm 43 is impressed as a negative bias voltage between the cathode and control electrode of the tube 24, thereby to determine the gain or signal current transmission efficiency of this amplifier tube. With no signal currents flowing in either of the two channels l4 and IS, the bias upon the control electrodes 32 and 33 of the tube 21 is determined solely by the drop across the adjustable cathode biasing resistor 38. Accordingly, the currents traversing the two parallel and-symmetrical current paths traced above are substantially equal. The resistor 38 is adjusted to a value such that the biasing voltage thereacross approaches the value required for anode current cut-off, whereby the tube 2'! will opreate as a half wave rectifier unit with respect to signal currents impressed upon the control electrodes thereof. As thus biased, the direct current traversing the two parallel current paths traced above is of relatively low value, whereby the bias voltages impressed upon the amplifier tubes l8 and 24 are relatively small. Accordingly, the tubes l8 and 24 normally operate with substantial gains therethrough so that both of the channels M and I phragm of the transmitter the resulting signal currents are transmitted through the coupling transformer I6 and the voltage divider I] to the input electrodes of theamplifier tube I8. The amplified signal currents are, in turn, transmitted through the coupling transformer I9 and the hybrid system |'3 to the line extending to the terminals I0. A portion of the amplified signal voltage appearing across the channel l4 at a point following the amplifier I8 is also impressed through the transformer 31 and the voltage dividing resistor 36 across the input electrodes 29 and 32 of the electron discharge tube 21. The section of this tube comprising the anode 30, the control electrode 32 and the cathode 29 operates to rectify this signal voltage. More particularly, the signal voltage has the effect of lowering the resistance of the space current path between the anode 30 and the cathode 23. As a consequence, the resistance of the direct current path including the resistor 4| and the resistor 43 is substantially lowered so that the direct current traversing this path is materially increased. The lowered resistance of this path also results in a decrease in the current traversing the parallel path comprising the resistance arm 44 due to the increased voltage drop across the portion 429. of the resistor 4|. Accordingly, the voltage drop appearing across the control section comprising the portion 42b of the resistor 4| and the resistance arm -43 is substantially increased, while the voltage drop across the control section of the resistor network comprising the portion 42b of the resistor 4| and the resistance arm 44 is I substantially decreased. The increased voltage drop across the control section comprising the arm 43 results in an increase in the negative potential of the control electrode of the tube 24, whereby the gain through this tube and, hence, the transmission efficiency of the channel I5 is substantially lowered. The decrease in the voltage drop across the control section comprising the resistance arm 44, on the other hand, results in a. decrease in the negative potential of the control electrode of the tube I8 with a resulting increase in the gain through this tube and a corresponding increase in the transmission efliciency of the channel H in use. By virtue of the decreased transmission efficiency of the channel are conditioned to transmit signal currents with I5, accomplished in the manner just explained,

signal currents transmitted through the hybrid system from the channel I4 to the input side of the channel l5 due to an unbalanced condition of the hybrid system are prevented from being reproduced at any substantial volume by the loud speaker l2. Thus, local oscillation of the system which would tend to produce interference known as singing is prevented and maximum side tone suppression is achieved.

It will be noted that a portion of the current traversing the space current path between the anode-30 and the cathode 29 is by-passed around the resistors 4| and 43 through the resistor 45. Thus, the latter resistor tends to lower the change in voltage which occurs across the control section comprising the resistor 43 in response to a given change in the resistance of the indicated space current path. The re- ,sistor 46, similarly, tends to lowerthe change in voltage across the resistance arm 44 which occurs in response to a given change in the resistance ofv the space current path between the anode 3| and the cathode 29. The two resistors 45 and 46 thus function as stabilizing resistors tov provide the desired ratio in voltagechanges produced in .response to a given'change in. the plate resistance of either section of the tube 21. At the end of each interval of speech 'when the signal voltage is removed from the sections comprising the arms 43 and 44 for a short period, thereby to prevent clipping or distortion of the signal currents and to prevent the production of transients which would be repro duced as noise. v q

The manner. in which the transmission efliciency of the channel I4 is decreased and that of the; channel I5 is increased when signal currents incoming over the line extending to the terminals ID are transmitted through the hybrid system to. the input side of the channel I5 is substantially similar to that just described with reference to the control of the channels efiected duringquse of the transmitter II. More particularly, incoming signal currents areimpressed through the hybrid system l3 and. the voltage dividing resistor 23 upon theinput electrodes of the amplifier tube 24, the amplified output signal currentsbeing delivered over an obvious path to the loud speaker I2 for reproduction. A portion of the signal voltage appearing across the channel I5 at a point following. the amplifier tube 24 is impressed through the transformer 35 and the voltage dividing resistor 34 across the input electrodes 29 and 33 of the tube 21. Asa result, the direct current resistance of the space current path between the anode BI and the cathode 29 issubstantially decreased so that the resistance of the current path comprising the resistance arm 44 is substantially lowered. A portion of the direct current normally traversing the opposite resistance arm 43 is thus diverted from this arm through the arm 44 and the space current path between the anode 3I and the cathode 29. Accordingly, the voltage drop across the control section comprising the arm 44 and the portion 42b of the resistor II is substantially increased to increase the bias upon the control electrode of the tube I18 and the voltage drop across the control section comprising; the arm 43 is substantially decreased to decrease the bias upon the control electrode. of the amplifier, tube 24. Thus, the gain 'of the tube I8 is lowered to decrease the signal current transmission efiiciencyof the channel I4,"while' the.:gain of the amplifier'tube 24 is simultaneously increased to raise the signal current transmission efficiency of the incoming signal current channel I5. By virtue of this arrangement, the soundgwaves developed through reproduction of the signal currents-bythe loud speaker -'I2 and transmitted acoustically to the transmitter II are prevented'from being transmitted with any substantial efficiency through the channel I4 to the" input side of the hybrid system I3. Accordingly, local oscillation of the substation and the transmission of echo currents are prevented. In this case also the-condensers 48 and 49 prevent an instantaneous change in the bias voltages across the two [control sections of the net- Work, '28, at the end of each interval of speech,

signal currents produced by such speech are transmitted through the transformer I6, the

and the cathode 29. Accordingly, a portion of the current flowing through the resistance arm 44 is diverted back to the arm 43 so that the voltage drop across the control section com-' prising this arm is increased and the voltage drop across the control section comprising the,

arm 44 is decreased. Thus, the gain through the amplifier tube I8 included in the channel,

I4 is increased and the gain through the amplifier tube 24 included in the channel I5 is decreased. The party conversing over the established connection at the distant end thereofis expected to stop speaking'in response to the conversation break-in, in which case the arrested signal current flow over the'channel' I5 results in an increase in the resistance of the space current path between the anode 3| and the cathode 23 such that the transmission efiiciency of the channel I4 is ,further increased, while the transmission efficiency of the channel I5 is furtherdecreased for reasons which will be apparent from the foregoing explanation. In a manner similar to that just described, if a subscriber at the substation terminating the distant end of the connection interrupts the subscriber using the apparatus illustrated while the latter subscriber is speaking, by speaking into the thereby to prevent clipping or distortion of the signal currents and the production of noise.

, If a. user of the substation circuit desires to break: in upon a speech train being reproduced by theloud speaker I2 before the train is ended,

he-may do by speaking directly into the microphone or transmitter II. The relatively large transmitter at the distant substation, the resulting fiow of signal currents over the line extending to the terminals Ill and through the channel l5 results in an increase in the'transmission efiiciency ofthis channel accompanied by a decrease in the transmission efliciency of the channel I4.

By properly adjusting the adjustable tap 42 along the resistor M, the ratio of the voltage changes across the control sections of the resistor network 28, respectively comprising the arms 43 and 44, in response to a given change in the' resistance of either of the space current paths through the tube 21 may be changed at will. Thus, considering the control section comprising the resistance arm 44 by way of example, it will be apparent that the voltage drop across this section is made up of two components, namely, that across the portion 42b of the resistor 4| and that across the arm 44.. Since the arm 42b is commonly included in the two control'sec- 'ti'ons, it will be apparent that an increase in the voltage across this resist or segment tends to produce an increase in the voltage across both control sections; While a decrease in the voltage across this resistor segment tends to produce a decrease in the voltage across both sections.

Accordingly, the larger the ratio of the resistance value of this resistor segment to the resistance values of the. arms 43 and 44, the smaller the change in voltage across each of the two control sections in response to agiven change in the resistance of either of the two space current paths of the tube 21. Conversely, the smaller the resistance of the segment 42b with respect to the resistance of the arms 43 and 44, the larger the change in voltage across the arms 43 and 44 in response to a given change in the resistance of either of the two space current paths. Since the magnitudes of the voltage drops across the arms 43 and 44 are changed in opposite senses in response to a change in either of the two space current paths, it will be apparent, therefore, that by changing the setting of the adjustable tap 42 the ratio of the voltage changes occurring in response to a given change in the resistance of either space current path may be varied at will. Accordingly, a simple expedient is provided for obtaining the desired change in the transmission efiiciencies of the two channels l4 and IS in response to signal currents traversing either of these two channels.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is contemplated to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What I claim is:

1. In a transmission system, a pair of channels for transmitting signal currents in difierent directions, electron discharge means including two space current paths and control electrodes for controlling the resistance of said paths, a control circuit including two parallel current paths each including a control section and one of said space current paths connected in series circuit relationship, means including a resistor commonly included in said parallel space current paths for varying the current traversing each of said control sections inversely in accordance with the resistance of the serially related space current path and directly in accordance with the resistance of the other space current path, means coupling said control electrodes to said channels so that the resistance of one of said space current paths is substantially changed when signal currents traverse one of said channels and the resistance of the other of said space current paths is substantially changed when signal currents traverse the other of said channels, means controlled in accordance with the current traversing one of said control sections for controlling the signal current transmission efiiciency of said one channel, and means controlled in accordance with the current traversing the other control section for controlling the signal current transmission efliciency of said other channel.

2. In a transmission system, a pair of channels for transmitting signal currents in different directions, electron discharge means including two space current paths and control electrodes for controlling the resistance of said paths, a control circuit including two parallel current paths each including a control section and one of said space current paths connected in series circuit relationship, means including a resistor commonly included in said parallel space current paths for varying the current traversing each of said control sections inversely in accordance with the resistance of the serially related space current path and directly in accordance with the resistance of the other space current path, means coupling said control electrodes to said channels so that the current traversing 'a first of said control sections is increased and the current traversing the second of said control sections is decreased when signal currents traverse one of said channels and the current traversing said first control section is decreased and the current traversing said second control section is increased when signal currents traverse the other of said channels, means for varying the signal current transmission efliciency of said one channel inversely in accordance with the magnitude of current traversing said second control section, and means for varying the signal current transmission efiiciency of said other channel inverselyin accordance with the magnitude of the current traversing said first control section.

3. In a transmission system, a pair of channels for transmitting signal currents in different directions, electron discharge means including two space current paths and control electrodes for controlling the resistance of said paths, a control circuit including two parallel current paths each including a control section and one of said space current paths connected in series circuit relationship, means including a resistor com monly included in said parallel space current paths for varying the current traversing each of said control sections inversely in accordance with the resistance of the serially related space current path and directly in accordance with the resistance of the other space current path, means coupling said control electrodes to said channels so that the resistance of one of said space current paths is substantially changed when signal currents traverse one of said channels and the resistance of the other of said space current paths is substantially changed when signal currents traverse the other of said channels, a first amplifier comprising an output circuit included in said one channel at a point preceding said coupling means, a second amplifier comprising an output circuit included in said other channel at a point preceding said coupling means, and means respectively connecting said control sections to said amplifiers so that the voltages thereacross respectively control the gains of said amplifiers.

4. In a transmission system, a pair of channels for transmitting signal currents in different directions, electron discharge means including two space current paths and control electrodes for controlling the resistance of said paths, a control circuit including two parallel current paths each including a control section and one of said space current paths connected in series circuit relationship, means including a resistor commonly included in said parallel space current paths for varying the current traversing each'of said control sections inversely in accordance with the resistance of the serially related space current path and directly in accordance with the resistance of the other space current path, means coupling said control electrodes to said channels so that the current traversinga first of said control sections is increased and the current traversing the second of said control sections is decreased wh'en signal currents traverse one of said channels and the current traversing said first control section is decreased and the current traversing said second control section is increased when signal currents traverse the other of said channels, a first amplifier comprising an output circuit included in said one channel at a point preceding said coupling means, a second amplifier comprising an output circuit included in said other channel at a point preceding said coupling means, means connecting said second control section to said'firs't amplifier so that the gain =of said first amplifier is varied inversely in accordance with the magnitude of current traversing said secondv control section, and means connecting said first control section to said second; amplifier so that the gain of said second amplifier is varied inversely in accordance with the current traversing said first control section.

5. In a transmission system, a pair of channels for transmitting signal currents in different directions, electron discharge means including two space; current paths and control electrodes for controlling the resistances of said paths, a control circuit including two parallel current paths each including a control section and one of said space current paths connected in series circuit relationship, said control'sections including a common resistor which is traversed by the space current traversing both of said space current paths, whereby the current traversing the noncommon portion of each of said control sections isvariable inversely in accordance with the resistance of the serially related space current path and directly inaccordance with the resistance of the other space current path, means for varying the portion of said resistor which is common to said. control sections, thereby to vary the ratio of the changes in voltages across said control sections resulting from a given change in the resistance of either of said space current paths,

means coupling said control electrodes to said channels so that the resistance of one of said space current paths is substantially changed when signal currents traverse one of said channels and the resistance of the other of said space current paths is substantially changed when signal currents traverse the other of said channels, means controlled in accordance with the voltage across one of said control sections for controlling thesignal current transmission efiiciency of "said one channel, and means controlled in accordance with the voltage across the other of said control sections for controlling the signal current transmission elficiency of said other channel.

6. In a transmission system, a pair of channels for transmitting signal currents in different directions, electron discharge means including two space current paths and control electrodes for controlling the resistances of said paths; a control circuit including two parallel current paths each including a control section and one of said space current paths connected in series circuit relationship, said control sections including a common resistor which is traversed by the space current traversing both of said space current paths, whereby the current traversing the noncommon portion of each of said control sections is variable inversely in accordance with the resistance of the serially related space current path and directlyoin accordance with the resistance of the other space current path, means for varying the portion of said resistor'which is common to said control sections, thereby to vary the ratio of the changes in voltages across said control sections resulting from a given change in the resistance of either of said space current paths, means coupling said control electrodes to said channels so that the current traversing a first of said control sections is increased and the current traversing the second of said control sections is decreased when signal currents traverse one of said channels and the current traversing said first control section is decreased and the current traversing said second control section is increased when signal currents traverse the other of said channels, means for varying the signal current transmission efficiency of said one channel inversely in accordance with the magnitude of current traversing said second control section, and means for varying the signal current transmission efiiciency of said other channel inversely in accordance with the magnitude control circuit including two parallel current paths each including a control section and one of said space current paths connected in series circuit relationship,-said control sections including a common resistor which is traversed by the space current traversing both of said space current paths, whereby the current traversing the noncommon portion of each of said control sections is-variableinversely in accordance with the resistance of the serially related space current path and directly in accordancewith the resistance of the other space current path, means for varying the portion of said resistor which is common to said control sections, thereby to vary the ratio of the changes in voltages across said control sections resulting fromv a given change in the resistance of either of said space current paths, means coupling said control electrodes to said channels so that the resistance of one of said space current paths is substantially changed when signal currents traverse one of said channels and the resistance of the other of said space current paths is substantially changed when signal currents traverse the other of said channels, a first amplifier'comprising an output circuit included in said one channel at a point preceding said coupling means, a second amplifier comprising an output circuit included in said other channel at a point preceding said coupling means, and means respectively connecting said controlsections to said amplifiers so that the voltages thereacross respectively control the gains of said amplifiers.

8. In a transmission system, a pair of channels for transmitting signal currents in different directions, electron discharge means including two space current paths and control electrodes for controlling the resistances of said paths, a control circuit including two parallel current paths each including a control section and one of said space current paths connected in series circuit relationship, said control sections including a common resistorwhich is traversed by the space current traversing both of said space current paths, whereby the current traversing the noncommon portion of each of said control sections is variable inversely in accordance with the I other channel at a point preceding said coupling means, means connecting said second control section to said first amplifier so that the gain of said first amplifier is varied inversely in accordance with the magnitude of current traversing said second control section, and means connecting said first control section to said second amplifier so that the gain of said second amplifier is varied inversely in accordance with the current traversing said first control section.

9. In a transmission system, a pair of channels for transmitting signal currents in different directions, electron discharge means including two space current paths and control electrodes for varying the resistances of said space current paths, .a Y connected resistance network, two parallel current paths commonly including the base leg of said resistance network and individually including said space current paths and the arms of said resistance network, means including the base leg of said network for varying the current traversing each of said arms inversely in accordance with the resistance of the serially related space current path and directly in accordance with the resistance of the other space current path, means controlledby the voltage across one of said arms and a portion of said base leg for controlling the signal current transmission efficiency of one of said channels, means controlled by the voltage across said portion of said base leg and the other of said arms for controlling the signal current transmission efiiciency of the other of said channels, and means coupling said control electrodes to difierent ones of said channels so that the resistance of one of said space current paths is changed when signal currents traverse said one channel and the resistance of the other space current path is changed when signal currents traverse said other channel.

10. In a transmission system, a pair of channels for transmitting signal currents in different directions, electron discharge means including two space current paths and control electrodes for varying the resistances of said space current paths, a Y connected resistance network, two parallel current paths commonly including the base leg of said resistance network and individually including said space current paths and the arms of said resistance network, means including the base leg of said network for varying the current traversing each of said arms inversely in accordance with the resistance of the serially related space current path and directly in accordance with the resistance of the other space current path, means controlled by the voltage across one of said arms and a portion of said base leg for controlling the signal current transmission efiiciency of one of said channels, means controlled by the voltage across said portion of said base leg and the other of said arms for controlling the signal current transmission efficiency of the other of said channels, means coupling said control electrodes to different ones of said channels so that the resistance of one of said space current paths is changed when signal currents traverse said one channel and the resistance of the other space current path is changed when signal currents traverse said other channel, and means for changing the portion of said base leg included in the paths from which said control voltages are derived, thereby to vary the ratio of the changes in said control voltages resulting from a given change in the resistance of either of said space current paths.

11. Telephone substation apparatus comprising transmitting means, receiving means and a hybrid system adapted to be connected to a line extending to said apparatus, a normally active channel for transmitting signal currents from said transmitting means to said hybrid system, a second normally active channel for transmitting signal currents from said hybrid system to said receiving means, electron discharge means including two space current paths and control electrodes for controlling the resistances of said paths, a control circuit including two parallel paths each including a control section and one of said space current paths connected in series circuit relationship, means including a resistor commonly included in said parallel space current paths for varying the current traversing each of said control sections inversely in accordance with the resistance of the serially related path and directly in accordance with the resistance of the other path, means coupling one of said control electrodes to one of said channels so that the resistance of one of said space current paths is substantially decreased when signal currents traverse said one channel, amplifying means controlled in accordance with the increased current flow through the control section serial related with said one space current path for increasing the signal transmission efficiency of said one channel and for decreasing the transmission efiiciency of the other of said channels, means coupling the other of said control electrodes to said other channel so that the resistance of the other of said space current paths is substantially decreased when signal currents traverse said other channel, said amplifying means being controlled in accordance with the increased current flow through the control section serially related with said other space current path to increase the signal transmission efiiciency of said other channel and to decrease the transmission elficiency of said one channel.

TURNER W. GILMAN. 

